This invention relates to a deterioration detecting system for detecting deterioration of a three-way catalyst of an internal combustion engine, and more particularly to a system of this kind which detects deterioration of a three-way catalyst based on outputs from two HC sensors arranged in an exhaust passage at locations upstream and downstream of the three-way catalyst, respectively, for detecting the concentration of hydrocarbons present in exhaust gases.
Conventionally a deterioration detecting system has been proposed e.g. by Japanese Patent Publication (Kokoku) No. 51-29260, which detects deterioration of a catalytic converter arranged in an exhaust system of an internal combustion engine by comparing outputs from two temperature sensors arranged in the exhaust passage at locations upstream and downstream of the catalytic converter, respectively.
Further, another deterioration detecting system has been proposed e.g. by U.S. Pat. No. 4,007,589, which detects deterioration of the catalytic converter by comparing outputs from two oxygen concentration sensors arranged in the exhaust passage at locations upstream and downstream of the catalytic converter, respectively.
However, neither of the above-proposed systems does not directly monitor the purifying performance of the catalytic converter, but detects temperatures of exhaust gases or values of concentration of oxygen present in same, to indirectly monitor the purifying performance of the catalytic converter by the use of the detected temperatures or values of concentration of oxygen, which makes it impossible to accurately detect deterioration of the catalytic converter.
Moreover, in the case of the former system using the temperature sensors, there is a time lag before the temperature of the catalytic converter is detected by the temperature sensor downstream of the catalytic converter, and hence there is a possibility of delayed detection of deterioration of the catalytic converter.
Further, in the case of the latter system using the oxygen concentration sensors, the oxygen concentration sensors have the output characteristic that normally the sensor output is inverted across the stoichiometric air-fuel ratio of an air-fuel mixture supplied to the engine. Therefore, the manner of detection of deterioration of the catalytic converter by the use of outputs from the oxygen concentration sensors is unavoidably complicated. Furthermore, the catalytic converter cannot be checked anytime but only under limited operating conditions of the engine.